The present invention relates to duplex communications systems, and to a method of establishing a digital time-division duplex radio communication link between one of a plurality of portable units and a base unit forming a cordless telephone system. The invention is directed particularly at the signalling protocol used in establishing the radio link from one of the portable units to the base unit, or vice versa.
Such a system is shown, in its simplest form, in FIG. 1 of the accompanying drawings to which reference will now be made. The system illustrated comprises a fixed part in the form of a base unit 1, and two portable units in the form of respective handsets 2, 3. Each handset comprises an earpiece, microphone and keypad, this latter being shown diagramatically under reference 4. In addition, each handset contains a respective radio transmitter/receiver (transceiver) and associated antenna 6, 7 by which the handsets may communicate with the base unit by radio, as represented by the dotted lines 8, 9. The base unit likewise contains a number of transceivers at least equal to the number of handsets, together with an antenna 5 for transmission and reception of radio signals from the handsets. The handsets may communicate with each other, but only via the base unit. The base unit also includes a hard-wired connection 10 to the external telephone system, and contains interface circuitry for interfacing the base unit transceiver to the external telephone line. Although only two handsets are shown, this is to be taken as an example of the simplest system and many more handsets, up to the capacity of the system, may be provided.
The present invent on is concerned with systems of the type illustrated in FIG. 1, in which the speech and other information to be transmitted between the base unit and the handsets s digitally encoded before transmission, is transmitted as a digital signal, and is decoded after reception to reproduce the original. A limited number of radio channels are allocated for the radio links 8, 9 and it is clearly therefore preferable to utilise the same channel for both ends of a radio link--i.e. duplex communication. Each transceiver in the system will be able to transmit and receive on a number of these channels, if not all.
In digital second generation (CT2) cordless telephone systems, burst mode duplex is used to provide full duplex speech on a single channel. This essentially means that each transmitter has to compress the encoded speech from a particular time interval called the burst period) down to just under half that interval (called the burst duration) in order to transmit the encoded speech and have time to receive the returning encoded speech in the other half of the burst period. This action is commonly called a ping-pong transmssion mode. It should be noted that the encoded speech corresponds to the speech from the entire burst period and on reception is expanded to its normal representation as continuous speech.
In order to provide a base unit capable of sustaining communication with a number of portable units it is desirable to synchronise the transmission bursts of the base unit. If this is not done, the phenomenon known as blocking can occur where a receiver which is trying to receive is sited close to a transmitter which is transmitting at the same time. This design goal of base burst synchronism and the use of the base unit as the synchronism master in burst mode duplex communication with a portable unit raises a problem. The problem is how a portable unit which cannot sense the burst synchronism of a target base unit can establish communication with that base unit as the base unit can only receive in predetermined burst windows.